Half-of-the-sites reactivity of inorganic pyrophosphatase from yeast is the result of induced asymmetry

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Baker’s yeast pyrophosphatase (EC 3.6.1.1) displays half-of-the-sites reactivity with respect to the inhibitors, phosphoric acid monoesters [ 1,2]. The enzyme catalyzes the hydrolysis and synthesis of inorganic pyrophosphate. It is a dimeric protein which consists of identical subunits, each containing an active centre. This enzyme is one of a growing list of proteins which display half-of-the-sites behaviour. The half-of-the-sites reactivity in proteins can be explained on the basis of a number of mechanisms [3]. Taking into account that the two subunits of the pyrophosphatase molecule are identical in primary structure and that the catalytic sites are distant from each other [4], the half-of-the-sites behaviour in pyrophosphatase is best explained in terms of the so-called pre-existing asymmetry or induced asymmetry models. In the former model, the oligomer consists of two conformationally different classes of subunits, which differ in their reactivity towards a modifying agent. According to the simple version of this model, whatever happens at one subunit has no influence on events that might occur on the neighbouring subunit. In the model of induced asymmetry, the subunits are conformationally identical in the native enzyme but modification of any subunit induces a conformational change in its neighbour with a resultant Half-of-the-sites reactivity